The invention relates to small-size fuel cells and particularly to biocatalytic direct alcohol fuel cells, referring to both hybrid and completely biocatalytic fuel cells. In a biocatalytic hybrid fuel cell, one of the anode or cathode chambers is substantially biocatalytic and the other chemical. Both parts of a completely biocatalytic fuel cell are substantially biocatalytic. More particularly, the invention relates to biocatalytic direct alcohol fuel cells, their use and a method of producing electric power using such a cell. The invention also relates to monitoring the state of such a fuel cell.
A biocatalytic fuel cell is a device that converts chemical energy directly to electricity. Suitable fuels, i.e. substrates, for cells of this type include different organic compounds, such as sugars and alcohols. The driving force of the cell is based on the redox reaction of the substrate when a live microorganism, cell-bound enzyme or isolated enzyme is used as catalyst. The operating principle of a biocatalytic cell is similar to that of a chemical fuel cell. The main difference is that the catalyst in a biological fuel cell is an enzyme (enzymes), and not a noble metal, such as platinum, and that the working conditions are mild. The temperature and pH of the solution are within the biological range, which refers to the operating conditions of the organism and enzyme(s).
Today, special attention is paid to energy sources that are suitable for portable, low-power electronic devices, such as mobile phones, computers, etc. The object is a better energy storage capacity and more environmentally friendly structural and material solutions than in batteries. Since the use of hydrogen as a fuel is not possible for safety reasons, direct-acting fuel cells (that work without a reformer) using logistic fuels have been the focus of interest. The drawback in chemical direct-acting fuel cells is that, to be powerful, they require intensive reaction conditions, such as a high temperature and strongly acidic or alkaline solutions. In most chemical fuel cells, an additional catalyst used is platinum or a platinum alloy, which is expensive and limitedly available. In addition, platinum is inactivated at very low carbon monoxide concentrations, carbon monoxide, in turn, being easily generated as a reaction product when any other fuel than pure hydrogen is used.
Biological fuel cells with an enzyme as the catalyst are previously known in the field. For instance, published U.S. Pat. No. 6,294,281 describes a biological fuel cell that uses human fluids or plant sap as the energy source and is therefore suitable for pacemakers implanted in a human, for example. According to the publication, an enzyme of a different type is disposed both on the anode and on the cathode.
US patent application publication 2002/0001739 discloses an electrically re-chargeable enzymatic battery. The fuel cell disclosed in the publication is not directly an alcohol fuel cell, but NADH (reduced nicotinamide adenine dinucleotide) is used as the fuel for providing the electrons. The enzyme, for instance dehydrogenases are mentioned as suitable, catalyzes the transfer of the electrons from the electron carrier, i.e. the NADH, to the mediator. Thus, the main reaction in the anode chamber is the oxidation of NADH. In addition, instead of the ion exchange membranes typically used in fuel cells, the publication employs a biologic proton pump, wherein the membrane is composed of a proton-transferring protein and the membrane is activated with a light diode.
Publication Enzymatic Fuel Cell: Biochemical Energy Conversion, Ranta, A., Zhang X-C. and Halme, A., Power Sources for the New Millenium, Proceedings of the International Symposium of ECS, 22-27.10.2000, Phoenix, USA, Proceedings Volume 2000-22, Ed. Ryan M. A. et al. The Electrochemical Society Inc., Pennington, USA, 2001, p. 108-117, discloses an enzymatic fuel cell, the fuel present in the anode chamber of the cell being subjected to enzymatic oxidation. The hydrogen ions, flowed through an ionselective membrane to the cathode chamber, react with atmospheric oxygen forming water. The enzyme is immobilized on the surface of the anode. Methanol can be used as the fuel in the cells, and either alcohol dehydrogenase or methanol dehydrogenase as the enzyme. The electrons released in the enzymatic reaction are transferred to the anode by means of the mediator. Phenazine methosulphate and phenazine ethosulphate are mentioned as suitable mediators.
Publication Study of Biological Fuel Cells, Halme, A., Zhang, X-C. and Ranta, A., Poster presentation at Small Fuel Cells 2000, New Orleans, USA, 26-28.4.2000, discloses an enzymatic fuel cell similar to the one in the above publication with the exception that the enzymatic oxidation of methanol takes place in the presence of coenzyme NAD+ (nicotinamide adenine dinucleotide).